The present invention relates to an adjuster strut for use in drum brake assemblies to compensate for wear in friction lining.
Adjuster struts are employed in drum brake assemblies in order to progressively adjust the separation between pairs of brake shoes as brake friction lining wears. A simplified example of a brake assembly that employs an adjuster strut is shown in FIG. 1, and this shows a drum brake assembly 1. The assembly 1 includes a pair of brake shoes 3 of T-shaped cross-section, an abutment 4 disposed between one pair of facing ends of the brake shoes 3 and about which the brake shoes can pivot, a hydraulic actuator 5 that applies an actuating force during brake actuation against the other pair of facing ends of the brake shoes 3, to move the brake shoes radially outwardly and into braking engagement with the internal braking surface of the drum 2, and an adjuster strut 6. The adjuster strut 6 is disposed generally horizontally in engagement with the inwardly depending web of one (or each) of the T-shaped brake shoes 3 and the web of the parking brake lever and in absence of an actuating force being applied to the brake shoes 3 by the hydraulic actuator 5, the respective webs engage either end of the strut 6 under the biasing influence of a return spring 7 and locate the shoes radially. Thus, the length of the strut 6 determines the radial spacing of the brake shoes 3 when no braking force is being applied. Through use, the friction lining of the brake shoes 3 wears over time and the adjuster strut 6 is lengthwise adjustable to increase the separation of the brake shoes and thus compensate for lining wear. It is the manner in which the adjuster strut is lengthened that the present invention has principal concern.
Various adjustable strut arrangements are known. Australian Patent No. 525459 in the name of The Bendix Corporation, discloses one form of an adjustable strut that employs a pawl and ratchet arrangement. This arrangement is complex and thus is relatively expensive to manufacture. Additionally, the relatively high number of parts that constitute the strut increases the potential for failure, as well as the cost of manufacture and necessity for regular maintenance.
A further strut arrangement is shown in European Publication No. 0262014, in the name of Bendix France and this strut suffers the same drawbacks associated with the above described known strut. This and the earlier known forms of adjuster strut are indicative of known struts in general, which typically are complex in construction.
European patent application EP 936 375 discloses a further strut arrangement that includes a rotatable wheel having internal and external parts. The respective parts are coaxial and are relatively rotatable in one direction of rotation and mutually rotatable in the opposite direction, by way of a clutch arrangement disposed between the parts. By this arrangement, the internal part can shift axially on a threaded shank to move the rotatable wheel, and by that movement, compensate for lining wear. This arrangement suffers from the drawbacks associated with the earlier discussed prior art in its complexity and cost to manufacture and its potential for failure and requirement for maintenance.
It is an object of the present invention to provide an adjuster strut which is of reduced complexity compared to known struts.
According to the present invention there is provided an adjuster strut for use in a drum brake assembly to shift the brake shoes of the assembly radially outwardly to as to compensate for wear in brake friction lining, said strut being of adjustable elongate extent and having axially extended and retracted conditions, said strut including engagement means disposed at opposite ends thereof for engaging opposed braking members of the brake assembly, a rotatable member which is rotatable about the lengthwise axis of said strut, an axially movable member which is movable axially upon forward rotation of said rotatable member to lengthen the elongate extent of said strut, in said retracted condition, and biasing means for biasing said strut toward said axially extended condition upon radial expansion of the brake shoes of said brake assembly during brake actuation, said rotatable member having an axially disposed face and a cam surface formed on said face, said cam surface defining a plurality of teeth formed in an annular ring, said strut including an elongate arm which is mounted for resilient pivoting movement and which includes a free end portion biased resiliently into engagement with said cam surface at least during axial extension of said strut, said arm being disposed at an oblique angle relative to said axially disposed face, wherein in use, said arm resiliently pivots during axial extension of said strut to said axially extended condition and causes said rotatable member to rotate in said forward direction, said biasing means being arranged to resist rotation of said rotatable member in a reverse direction opposite to said forward direction upon said strut moving from said axially extended condition to said axially retracted condition during radial contraction of said brake shoes, so that the elongate extent of said strut in said axially extended condition is not reduced as said strut moves to said axially retracted condition.
For the purposes of this specification, the expression xe2x80x9cbraking membersxe2x80x9d is to be understood as including the opposed brake shoes of a brake assembly and if provided, the parking brake lever of the assembly.
In one arrangement, upon separation of braking members during brake actuation, the rotatable and axially movable members are shifted by the biasing means to an axially extended position, and the arm resiliently pivots from a rest or home position in a forward direction and by that pivoting movement, the free end of the arm rotates the rotatable member in the forward direction to relatively extend the axially movable member. Upon return movement of the braking members the rotatable and axially movable members retract axially from the axially extended position, while the arm returns to the rest position which causes the free end of the arm to shift relative to the cam surface in the reverse direction. Upon sufficient shifting movement (upon sufficient lining wear), the free end moves past one tooth of the cam surface to a position adjacent a circumferentially spaced tooth. In this arrangement, each time the arm is positioned adjacent a new tooth, it is positioned to catch or engage that new tooth and to rotate the rotatable member forward by that engagement during radial expansion of the brake shoes.
In most arrangements, the arm can be arranged to operate either in tension or compression. In the above arrangement the arm typically would undergo tensile loading when the axially movable member is shifted axially outwardly to extend the length of the strut and that tensile loading would cause the arm to pivot and rotate the rotatable member in the forward direction. However, it equally is permissible that the arm be arranged for compression loading to rotate the rotatable member and in that arrangement, rotation of the rotatable member occurs during return axial movement of the axially movable member. In this latter respect, the arm may pivot and thus the free end portion thereof may shift relative to the cam surface during outward axial movement of the axially movable member, without rotating the rotatable member in the forward direction. However, if that shifting movement is sufficient, the free end portion may engage a tooth of the cam surface, such that upon return axial movement of the axially movable member, with the arm in compression, the arm may rotate the rotatable member in the forward direction and thus adjustably lengthen the strut as required.
The arrangement of the arm to act either in tension or compression is considered to be a matter of design choice. It is therefore to be appreciated that where reference is made in this description to a particular arm arrangement, the likelihood is that the arm could be arranged to operate in the desired manner either in tension or compression.
In one form of the invention, the arm is mounted to the strut, such as to the body of the strut, or to a bracket connected to the body, and the arm extends in the manner required into engagement with the cam surface of the rotatable member. Alternatively, the arm may be fixed to or depend from a part of the brake assembly to which the adjuster strut is applied. In one arrangement, the arm may be mounted by attachment to the radially inwardly depending web of a T-shaped brake shoe. Thus, the actual mounting position of the arm can vary, as long as the reaction of the free end of the arm with the cam surface of the rotatable member is maintained as defined. In this respect, it is necessary that the mounting position of the arm is one which experiences movement relative to the rotatable body, so as to cause the arm to pivot and rotate the rotatable member during brake actuation, or alternatively, during brake release (depending on the manner in which the arm is arranged to operate).
It is preferable that the arm of the strut be formed from a flat plate and that the free end be sufficiently hard to resist wear during engagement with the cam surface. The free end may be hardened for that purpose. The arm is preferably mounted obliquely to the axis of the strut so that the free end of the arm can cause the rotatable member to rotate upon axial extension of said strut.
The rotatable member is preferably of circular cross-section and the teeth defined by the cam surface are preferably defined about the periphery of the axial face of the rotatable member. Each tooth may present an inclined surface in at least the forward direction of rotation of the rotatable member, although it is preferable that the teeth are formed to have inclined surfaces on both sides thereof diverging or converging in opposite directions. Those sides preferably converge to a pointed apex and the inclined sides between adjacent teeth preferably intersect to define an inverted and pointed apex. If the teeth of the cam surfaces are formed with inclined opposite faces, the free end of the arm can be arranged to slide down the rear face and further rotate the rotatable member during return axial movement of the rotatable and axially movable members.
The teeth of the rotatable member can have any suitable pitch and height, and the arm can have any suitable angle of attack relative to the axial face of the rotatable member, dependent on the rate of change of axial extension and the desired axial travel of the axially movable member to compensate for friction lining wear. The shape and size of each tooth may be arranged so as to not result in axial extension of the axially movable member relative to the rotatable member upon every brake actuation. Indeed that arrangement is preferred, as the brake assembly is typically required to be applied a substantial number of times before wear of the friction lining becomes sufficient to require compensation by way of an incremental tooth rotation of the rotatable member. Thus, while the arm may rotate the rotatable member upon brake actuation, the free end of the arm may only travel fully past one tooth and reposition between a new pair of adjacent teeth, after as many brake applications are applied as are required for the friction lining to wear an amount sufficient to require compensation. If the arm free end does not fully increment past a tooth to reposition between a new pair of adjacent teeth, it may rest between brake applications, on the tooth surface, or on the cam surface between adjacent teeth. For example, if the teeth have oppositely inclined converging faces, and if the arm free end does not slide fully over the forward facing inclined tooth surface and reposition between a new pair of adjacent inclined surfaces, it may rest between brake applications, on the first mentioned inclined surface between the base and the peak thereof. The arm free end will rest at or near to the base between adjacent teeth when it has just incremented past a tooth, and can rest against the inclined surface of a tooth closer to the peak thereof when the lining has worn, but when the wear is not enough for the arm to increment past the peak.
In circumstances when the arm rests against an inclined forward facing surface of a tooth between brake applications, the arm, by its resilient bias, may apply a force tending to cause the rotatable member to rotate in the reverse direction. However, the invention includes means resisting or limiting that movement such that the overall movement provides for a net forward shift in the rotatable member regardless of any reverse movement. The biasing means, performs this function as hereinafter described.
The arrangement of the arm and the biasing means is such as to apply frictional resistance against movement of the rotatable member in the reverse direction, but to allow movement in the forward direction. Because the free end of the arm engages the cam surface in an oblique manner and catches a tooth only for forward rotation thereof, the force applied to the rotatable member by the arm in the forward direction is greater than in the reverse direction. The biasing means therefore applies a force resisting rotation of the rotatable member, that exceeds any force applied by the arm to the rotatable member in the reverse rotating direction, but which is less than the force applied by the arm to the rotatable member in the forward rotating direction. The resistance to reverse rotation permits the free end of the arm to shift in a net forward movement relative to the cam surface upon separation or return movement of the braking members preferably without any, or at most minor reverse rotation of the rotatable member. As discussed, if the free end does not increment fully over and past the highest point (relative to the arm swing movement) of the tooth profile, it may rest against that profile surface until further and sufficient lining wear takes place. Given that a substantial number of brake applications may occur before lining wear becomes sufficient for wear compensation, the free end rest position between brake actuations may slowly creep or progress up the inclined surface over successive brake applications.
In each of the embodiments described, it is the cooperation between the arm and the teeth of the cam surface that promotes rotation of the nut in the forward direction. Without the teeth, the arm would not xe2x80x9ccatchxe2x80x9d the cam surface and cause it to rotate. The invention is not restricted to any particular form of tooth profile, as long as the above-mentioned cooperation takes place.